FIG. 1 illustrates a conventional ion source apparatus using a laser beam disclosed, for example in Japanese Patent Laid Open Gazette No. 50-22999. In figure reference numeral 1 designates a particle flow generator for generating a material to be ionized in the form of atomic flow or molecular flow, reference numeral 2 designates an exhaust nozzle of the particle flow generator 1, reference numeral 3 designates an atomic beam drawn out from the exhaust nozzle 2, reference numeral 4 designates three dye laser apparatuses, reference numeral 5 designates lenses for converging the laser beams from the dye laser apparatuses 4 with the atomic beam 3 at a point P, reference numeral 6 designates an atomic beam in which one part of the atomic beam 3 is ionized by irradiation of the laser beam and reference number 7 designates electrodes for selecting only ions from the atomic beam including ions.
The conventional ion source using the laser beam includes those described above. A description is given of an operation when, for example Na (sodium) is put in the particle flow generator 1 and the atomic beam 3 of Na is generated and then is ionized by the laser beam.
When the atomic beam 3 of Na from the exhaust nozzle 2 of the particle flow generator 1 passes through the point P at a certain speed, the laser beams from the dye laser apparatuses 4a,4b and 4c are converged by the lenses 5a, 5b and 5c at the point P.
Since an energy level diagram of the Na atom is as shown in FIG. 2, when a wavelength (.lambda.a) of the first dye laser apparatus 4a is 589 nm and a wavelength (.lambda.b) of the second dye laser apparatus 4b is 568.8 nm, the Na atoms at the point P are optically excited from a ground state of 3s .sup.2 S.sub.1/2 to a 4d state through a state of 3p .sup.2 P.sub.3/2 by the laser beam. Since the 4d state of the Na atoms is at a distance of approximately 7000 cm.sup.-1 from an ionization limit of the NA atoms, when a wavelength (.lambda.c) of the third dye laser apparatus 4c is 1.4 .mu.m or less, the Na atoms are directly ionized by the laser beam and ions are partially included in the atomic beam which passed through the point P. When an electric field is applied to the atomic beam 6 including the ions by the electrodes 7, only ions are deflected and irradiated to a predetermined region as an ion beam.
According to the conventional ion source using the laser beam described above, when the wavelength band width of the laser beam is equal to an absorption wavelength band width of each transition, a power density of the first dye laser beam required when the Na atoms irradiated by the laser beam at the point P are effectively ionized is approximately 10 W/cm.sup.2 or more because Einstein's A coefficient of 3s .sup.2 S.sub.1/2 -3p .sup.2 P.sub.3/2 transition (a transition wavelength is 589 nm) of Na is approximately 6.3.times.10.sup.7 (1/s), a required power density of the second dye laser beam is approximately 40 W/cm.sup.2 or more because Einstein's A coefficient of 3p .sup.2 P-4d transition (a transition wavelength is 568.8 nm) is approximately 1.3.times.10.sup.7 (1/s) and a required power density of the third dye laser beam is 10.sup.7 W/cm.sup.2 or more because an absorption cross section of the beam when Na in the 4d state is directly ionized is 10.sup.-18 cm.sup.2 or less.
According to the conventional ion source using the laser beam, because the absorption cross section of the beam in the case of direct ionization is 10.sup.-18 cm.sup.2 or less, the dye laser beam has to have a large output of 10.sup.7 W/cm.sup.2 or more as a laser beam power density. However, an output of a large size dye laser with a high output, which is available in general, is approximately 10.sup.6 W in the case of a pulse laser and the output is approximately IW in the case of a continuously oscillating laser. Therefore, it is necessary to converge the laser beam to a small region of 10.sup.-1 cm.sup.2 or less in case of the pulse laser and 10.sup.-7 cm.sup.2 or less in case of the continuously oscillating laser to increase the laser beam power density.
Therefore, ions cannot be sufficiently obtained because a region for ionization is small.
In addition, even if a density of atoms in the atomic beam 3 is increased in order to increase an amount of ions, when the ion density is 10.sup.10 /cm.sup.3 or more, a space-charge electric field of the ion itself becomes 3 kV/cm or more, so that ions are spread in the region of the atomic beam 6 including ions after the point P. Therefore, an amount of ions, which effectively reaches the electrodes 7 and enters the predetermined region, is not increased. As a result, it is found that the amount of ions cannot be largely obtained even if the density of atoms in the atomic beam 3 is increased.
Furthermore, when atoms or molecules other than alkali metal such as Na or alkaline earth metal such as Ca (calcium) are ionized, the wavelength .lambda.a of the first dye laser apparatus 4a has to be a wavelength ranging from an ultraviolet region of approximately 400 nm or less to a vacuum ultraviolet region of approximately 100 nm. However, a lower limit of the oscillation wavelength of the generally available dye laser is 200 nm, so that a material which can be ionized is limited.